1. Field of the Invention
The invention relates to the polishing of glasses, semiconductors, dielectric/metal composites and integrated circuits. More particularly, this invention relates to improvements in the surface preparation of composite materials where improved differences in rate between silica and other components are desired.
2. Description of the Prior Art
Conventional polishing compositions or slurries generally consist of a solution which contains abrasive particles. The part, or substrate is bathed or rinsed in the slurry while an elastomeric pad is pressed against the substrate and rotated so that the slurry particles are pressed against the substrate under load. The lateral motion of the pad causes the slurry particles to move across the substrate surface, resulting in wear, volumetric removal of the substrate surface.
In many cases the rate of surface removal is determined solely by the degree of applied pressure, the velocity of pad rotation and the chemical activity of the slurry particle. Thus, as reviewed by Cook [J. Non-Cryst. Solids, 1990], slurry particles with a high degree of chemical activity toward the substrate (e.g., CeO.sub.2 toward SiO.sub.2) show significantly higher polishing rates than more inert particles (e.g. La.sub.2 O.sub.3 toward SiO.sub.2). This enhancement of chemical activity of the polishing particle has been the basis of numerous patents, for example U.S. Pat. No. 4,959,113.
An alternative means of increasing polishing rates is to add components to the slurries which by themselves are corrosive to the substrate. When used together with abrasive particles, substantially higher polishing rates may be achieved. This process, often termed chemo-mechanical polishing (CMP) is a preferred technique for polishing of semiconductors and semiconductor devices, particularly integrated circuits. Examples of such CMP processes for enhanced polishing of silicon wafer surfaces have been disclosed by Payne in U.S. Pat. No. 4,169,337. Beyer et al. (U.S. Pat. No. 4,944,836) and Chow et al. (U.S. Pat. No. 4,702,792) teach the utility of CMP in improving rate selectivity in the polishing of dielectric/metal composite structures such as interconnect vias in integrated circuit structures. Specifically they teach the introduction of additives which accelerate dissolution of the metal component. The purpose of this and other related techniques is to preferentially remove the metal portion of the circuit so that the resulting surface becomes coplanar. The process is ordinarily termed planarization.
It is highly desirable to improve the selectivity of metal planarization as much as possible. Carr et al. (U.S. Pat. No. 4,954,142) teach further improvements in CMP planarization of dielectric/metal composite structures by addition of a chelating agent to the slurry which is selective for the metal component of interest. This results in a further increase of the corrosion rate of the metal phase and increase selectivity of metal versus dielectric phase removal, making the planarization process much more efficient.
A number of anions have been demonstrated to chelate or complex with Si.sup.4+ in such a manner as to accelerate corrosion of silica or silicate materials. As described by Bacon and Raggon [J. Amer. Ceram. Soc. vol. 42, pp.199-205, 1959] a variety of weak acids were shown to accelerate the corrosion of silica and silicate glasses in neutral solution (pH.about.7). The effect was ascribed to the ability of the free anions of the acid (conjugate base) to complex the Si.sup.4+ cation in much the same manner as the pyrocatechol-silicate complexes described by Rosenheim et al. (A. Rosenheim, B. Raibmann, G. Schendel; Z. anorg. u. allgem. Chem., vol. 196, pp. 160-76, 1931] as shown below: ##STR1##
The corrosive anions described by Bacon and Raggon all had similar structures which were in turn closely similar to pyrocatechol (1,2-dihydroxybenzene), namely, all were mono or dicarboxylic acids which had hydroxyl groups at secondary or tertiary carbon sites which were located at an alpha position with respect to the carboxylic acid group. An example of an active versus an inactive compound is shown below:
HOOC--CHOH--CHOH--COOH: Tartaric acid (active) pKa.sub.1 =3.02 PA1 HOOC--CH.sub.2 -CH.sub.2 --COOH: Succinic acid (inactive) pKa.sub.1 =4.2 The pKa is the logarithm of the association constant Ka for formation of the free anion, as defined by the reaction: ##STR2## Thus a lower pKa indicates a stonger acid. At equivalent pH a higher conjugate base concentration is found in solution. PA1 Tartaric acid: pKa.sub.1 =3.02 PA1 Citric acid: pKa.sub.1 =3.1 PA1 Phthalic acid: pKa.sub.1 =2.95
versus
Prior art corrosion literature also describes the corrosive effects of catechol in static solution. As shown by Ernsberger (J. Amer. Ceram. Sec., vol. 42, pp.373-5, 1959), addition of pyrocatechol to Ethylene Diamine Tetraacetic Acid (EDTA) solution produces enhanced corrosion of soda-lime-silicate glass in the pH range 10-14. The enhancement was significant with rates at least twice as high as with EDTA alone in the solution. A maximum effect was found at pH 12.5. Once again, the effect was attributed to complexation of free Si.sup.4+ cation with the catechol.
From the above, it is clear that published literature on the subject indicates that such additives have been shown to be corrosive to Silica or silicates under static exposure. The mode of the corrosion is believed to be the formation of a complex or chelate with free Si.sup.4+ cations. Thus, in like manner to the teaching of U.S. Pat. No. 4,954,142, a higher silica removal rate during polishing would be expected when such additives are present in the polishing solution. Consequently, these types of additives have never been used in metal planarization, as metal/silica selectivity was expected to be seriously degraded.
While the prior art CMP procedures described above appear attractive, they possess significant drawbacks. Specifically, the etchants incorporated into prior art CMP slurries are isotropic, i.e., they attack all portions of the exposed phase, regardless of position. Thus significant incorporation of etchants into CMP slurries often results in increases in surface roughness and texture when recessed features become etched. In the polishing of integrated circuits this effect is termed dishing and often occurs at the end of the process when a significant portion of the substrate surface is composed of the more durable component. It is highly undesirable, as the object of polishing is to produce a uniform plane surface free from texture.
It is clear from the above discussion that if the polishing rate of the silica phase of a composite structure could be reduced in a controlled manner, selectivity could be significantly improved. This would allow use of solutions which are less aggressive to the other (metal) phase, thus permitting efficient CMP processing of metal/silica composites with reduced dishing.
Accordingly, it is the object of this invention to provide a solution for polishing silicon, silica, silicon- or silica-containing articles wherein the polishing rate of the silicon or silica phase is modulated or controlled by the addition of specific additive or complexing agents.
It is also the object of this invention to provide an improved polishing slurry and polishing method for composite articles which results in improved polishing slurry and polishing method for composite articles which results in improved selectivity during the polishing process, particularly for metal dielectric composites such as those occurring in integrated circuit structures.
These and other objects of the invention will become apparent to those skilled in the art after referring to the following description and examples.